1. Field of the Invention
The present invention relates to a solid-state imaging device, a fabrication method thereof, an imaging apparatus, and a fabrication method of an anti-reflection structure.
2. Description of the Related Art
In solid-state imaging devices, it is necessary for a photoelectric conversion portion to convert incident light to electrical signals while preventing reflection of the incident light in order to increase conversion efficiency of the photoelectric conversion portion performing photoelectric conversion on the incident light.
Therefore, it is desirable to decrease the light components reflected from the interfaces as much as possible.
In solid-state image sensors, a light-condensing stacked structure is formed to efficiently condense light. In this case, since the stacked layers of different materials produce an interface at which a difference in refractive indices is large, there occurs an optical loss due to interfacial reflection, which may lead to a reduction in sensitivity if no countermeasures are taken. In addition, the light reflected from the interface may become the source of noise such as flare or ghost.
In this regard, a method has been proposed to form an anti-reflection film on an interface at which a difference in refractive indices is large so as to reduce interfacial reflection (for example, see JP-A-2007-242697 and JP-A-6-292206).
As a high performance anti-reflection structure, a method has been proposed to form a relief structure on an on-chip lens, for example, so as to reduce the interfacial reflection (for example, see JP-A-2004-47682, JP-A-2006-147991, and WO2005/109042).
The reduced interfacial reflection results in the reduction in noise light such as flare or ghost which is generated when light reflected once is reflected again from another member such as a protective glass and then enters the lens.
According to the method to form the anti-reflection film on the interface at which a difference in refractive indices is large so as to reduce interfacial reflection (for example, JP-A-2007-242697 and JP-A-6-292206), since the anti-reflection structure is achieved with a single layer, anti-reflection performance can be increased by choosing a film thickness such that the phases of light are reversed therein.
However, in a practical fabrication of solid-state image sensors, since a step or the like exists between a light-receiving element portion and a peripheral circuit, it is difficult to form a uniform single-layer film on the light-receiving elements. Therefore, an interference state differs for each location of the light-receiving elements. Moreover, since the optimum thickness of the anti-reflection film generally differs depending on the wavelength of visible light, an uneven thickness may become the cause of color unevenness.
From these and other reasons, the use of a single-layer anti-reflection structure in a solid-state image sensor has drawbacks in fabrication and principle.
For detailed description, an appearance of an interference pattern caused by reflections when a silicon nitride film is formed as an anti-reflection film on a silicon-air interface (a silicon-side interface) is illustrated in FIG. 1.
The thickness of the anti-reflection film is optimized to a 560 nm wavelength of visible light. For this reason, the interference pattern observed with incident light of 560 nm wavelength disappears almost completely, and it can thus be understood that the film functions properly as an anti-reflection film. On the other hand, the interference pattern observed with incident light of 440 nm wavelength does not disappear much. The optimum thickness of the anti-reflection film where the anti-reflection film has an interference reduction effect changes depending on the wavelength of incident light. Therefore, since the sensitivity to incident light also varies with a variation in the thickness of the anti-reflection film, such a sensitivity variation becomes the cause of color unevenness.
As an anti-reflection method, there are known anti-reflection films that use single-layer or multi-layer interference coatings. Although these films exhibit excellent anti-reflection properties at a particular wavelength band, it is particularly difficult to form an anti-reflection film having excellent anti-reflection properties over all the visible light bands.
Moreover, it is also difficult to have anti-reflection capabilities for light coming at every angle of incidence.
Furthermore, the anti-reflection capabilities of these anti-reflection films are sensitive to their film thickness. In addition, in order to maintain stable anti-reflection properties, many problems such as difficulties in fabrication management should be solved.
A method has been proposed to provide an anti-reflection structure which is constructed from microscopic protrusions formed on an interface of a solid-state image sensor, at which refractive indices are different, thus preventing reflections (for example, see JP-A-2004-47682, WO2005/109042, or JP-A-2006-332433). In the anti-reflection structure using protrusions, it is considered preferable for the microscopic protrusion to have a size about half the wavelength of incident light, and the size is about 200 nm for visible light. Thus, a stable formation method is difficult.
In the technique disclosed in JP-A-2004-47682, 100 nm-size patterns spaced by 100 nm from adjacent patterns are formed by electron beam exposure, and the patterns are subjected to dry-etching, thus forming a protrusion pattern. In the technique disclosed in WO2005/109042, a protrusion pattern is formed by using any one of a combination of photolithography and hot-reflow, a combination of nickel-electrocasting and replica molding, and dual-beam interference exposure. In the technique disclosed in JP-A-2006-332433, a protrusion pattern is formed by forming a coating film using aluminum compound, which is then subjected to hot-water treatment or vapor treatment. However, neither of the methods can be said to be a low-cost and highly reliable formation method of an anti-reflection structure using protrusions. In addition, neither of the disclosures teaches formation of a spindle shape suitable for an anti-reflection structure.
As an additional method, a method has been proposed to form a 125 nm-size resist pattern on a metal film by electron beam exposure and etch both the metal film and the glass substrate, thereby obtaining a conical or pyramidal shape (for example, see JP-A-2001-272505). However, it is expensive to form a microscopic resist pattern by electron beam exposure. Moreover, this disclosure does not teach details of a method of obtaining a spindle shape suitable for an anti-reflection structure.
As another method, a method has been proposed to perform microscopic processing by performing etching using nano-size particles as a mask (for example, see JP-A-2001-272505 and U.S. Pat. No. 4,407,695). However, in the method disclosed in JP-A-2001-272505 and U.S. Pat. No. 4,407,695, it is difficult to form a spindle shape which is the shape of a protrusion pattern suitable for an anti-reflection structure. The method disclosed in JP-A-2001-272505 teaches formation of only nano-size columnar or conical shapes. In the method disclosed in U.S. Pat. No. 4,407,695, it is possible to form an oval hole shape, but it is difficult to form a spindle shape.
The reason why the shape of the protrusion pattern suitable for the anti-reflection structure is a spindle shape will be described briefly with reference to FIG. 2. As illustrated in FIG. 2, since light reflections are caused by an abrupt change in refractive index, by forming a structure such that refractive indices are distributed continuously at an interface of different substances with a protrusion pattern, it is possible to reduce the light reflections. When a widthwise dimension of the protrusion pattern is smaller than a wavelength of light, a spatial occupancy of a substance (e.g., air) on one side of the interface changes gradually so that the substance is switched to a substrate (e.g., a microlens) on the other side of the interface, whereby an effective refractive index changes continuously.
Since a variation in spatial occupancy has the same meaning as a volume variation between the substances on both sides of the interface, a spindle-shaped anti-reflection structure is suitable which has a sinusoidal curved surface where a volume variation is smooth as illustrated in FIG. 3.
However, neither of the disclosures presents a method of stably forming a protrusion pattern suitable for an anti-reflection structure. Moreover, for example, in a method in which a protrusion pattern is formed on a passivation film, and a color filter layer is formed on the passivation film, there is a high possibility of the protrusion pattern being deformed at the time of coating a color filter material. Thus, such a method cannot be said to be a practical method.
The solid-state image sensor disclosed in JP-A-2004-47682 has an anti-reflection structure which has a relief structure and is formed on the surface of a passivation film. The relief structure is characterized in that it has an aspect ratio of 1 or higher, which appears at intervals of 0.05 μm to 1 μm. According to the studies of the present inventors, when the relief structure is formed to have an aspect ratio exceeding a certain level, a reduction in sensitivity was observed. This is thought to result from an increase in an optical path length of a light condensing structure with the increase in the aspect ratio of the relief structure.
According to the formation method disclosed in JP-A-2004-47682, patterns which are 100 nm in width are patterned by electron beam lithography so that the patterns are spaced by 100 nm from adjacent patterns. Thereafter, reactive ion etching (RIE) is performed, whereby protrusion patterns are formed. In this formation method, when the protrusion patterns are arranged on light-receiving elements disposed at a pitch of 2.0 μm, 400 protrusion patterns are required for each light-receiving element.
In recent solid-state image sensors, it is common that more than one million light-receiving elements are mounted on one chip. In this case, 400 million protrusion patterns are required for each chip. When such many patterns are formed by electron beam lithography, if a rendering time is 100 nsec per one protrusion pattern, it may take 11 hours or longer for one sheet of 300-mm wafer and it is thus not practical.
Moreover, the relief structure disclosed in JP-A-2006-147991 is fabricated by lithography so that relief structures have a height ranging from 100 Å to 5000 Å and are arranged at a pitch such that an incident light is not diffracted. However, this disclosure does not teach the details of the fabrication method using lithography.
Moreover, the relief structure disclosed in WO2005/109042 is characterized in that relief structure units have a pitch and a height, satisfying an expression 0.1λ<pitch<0.8λ and an expression 0.5λ<height<5λ (where λ is a wavelength of an incident light).
However, when pitch=0.11λ and height=4.4λ, an aspect ratio will become 40, which will lead to a sensitivity reduction as described above. Moreover, such a structure is not practical from the perspective of preventing shading (which is a phenomenon where the light collecting characteristics of pixels disposed on the border of a light-receiving region where light enters at oblique angles are more degraded than pixels disposed on the center of an optical axis).
Although it is described that the structure can be fabricated by a nanoimprinting method, when relief structure units of which an aspect ratio is high (namely, the structure is tall) are fabricated, convex parts are hard to separate from a mold. Therefore, there is a problem with the separability of a mold, and it is not practical.